The present invention relates generally to matrix-type flat panel display systems, and more specifically to liquid crystal display systems and, still more specifically, to gas discharge addressed matrix-type liquid crystal display panels and, still more specifically, to matrix-type liquid crystal displays with improved gray-scale and/or color.
The invention is particularly directed to a matrix-type flat panel display comprised of an array of rows and columns of pixel elements and in which the brightness level of each pixel element can be modulated by modulating the physical size or area of each pixel element.
Two of the primary problems in electronic displays is that of addressing a large array of pixels and achieving gray-scale and color. Direct matrix addressing using row and column lines to identify and select each pixel is the simplest addressing technique; however, it inherently possesses cross-coupling such that (at a minimum) one-third of the addressing signal to turn on a pixel is applied to all off-pixels. Some display materials, such as gas discharge or light emitting diodes (LEDs) have a very high discrimination ratio on an addressable matrix; that is to say, that one-third cross-coupling voltage is of no consequence. Other display materials have strong degree of required non-linearities, but not strong enough to be addressed in large matrices.
The degree of non-linearity is directly proportional to the number of rows scanned. Examples of materials are liquid crystal (LC) and electroluminescent (EL). To use these technologies, diodes, transistors or other highly non-linear elements are placed in series with the material to make an active matrix addressed display. In a typical active-matrix flat panel display, a semi-conductor process is used in making the display switches. Thin film transistors (TFT) active matrix and gas discharge switched pixel arrays have been suggested in the past. Their costs are intimately tied-up with yields and the proprietary manufacturing technologies. For thin film transistors, it is difficult to fabricate large quantities and large sizes with high production yields.
The present invention relates generally to a matrix-type flat panel display in which an AC plasma gas discharge system uses spatial modulation to control the gray-scale of a liquid crystal layer. Preferably the liquid crystal medium is one which is operable in an on-off (bi-level) mode where the total area of saturation is directly determined by the spatial area charged by the gas discharge contiguous or adjacent thereto. A charge storage surface such as a dielectric layer between a transparent electrode array, the LC medium and the gas medium stores a charge which is caused to spread in proportion to the amplitude of conjoint voltages at selected matrix cross-point. The charge spread area establishes a spatial or area size of the spot where the liquid crystal material changes state thereby providing spatial gray level of light transmission at the selected matrix cross-points. By using three Color channels (red, blue, green, for example), low cost, large screen multi-color display are attained.
A preferred embodiment of the invention comprises a layer of a liquid crystal (LC) display medium and a gas discharge matrix defined by rows and columns of discharge sites arranged and configured to areawise charge a matrix of area of a corresponding matrix of display pixel sites in the LC layer. The LC layer has viewing and non-viewing sides and a source of light illuminating the matrix of display pixel sites from the non-viewing side. The gas discharge matrix is of the AC discharge type wherein a first transparent substrate has a first electrode array thereon. A protective coating on the first electrode array is preferably a high resistance coating so as to isolate the first electrode array from the gas.
An array of non-conductive rib means defines channels or grooves filled with a gas discharge medium. A Penning mixture, such as neon and trace of a minority gas (argon, krypton, xenon, etc) and mixtures thereof. Light production by the discharge is not important. What is important is the formation of charges for storage on the discharge storage surface. The gas discharge medium fills the space between the array of ribs and a thin glass layer on the ribs serves as the dielectric charge storage surface, the size of the area of charge stored on the surface being a function of the amplitude of conjoint voltages and controls the spatial size of the spot where the liquid crystal material changes state.
The transparent layer of LC medium is preferably on the thin glass layer opposite the side contacting the array of ribs and, as noted above, the LC medium is, in the preferred embodiment, operated in an on-off (bi-level) mode where the total area of saturation is directly determined by the spatial area of the gas discharge adjacent thereto. A second transparent member or substrate, having an outer viewing surface and electrode bearing opposite surface has the transparent electrode array on the electrode bearing surface and the transparent electrode array is oriented transverse to the direction of the first electrode array so that the gas channels are aligned with the transparent electrodes. This alignment allows the discharge to spread (within predetermined limits to avoid cross-talk) and by adjusting the voltage, the charge on the dielectric layer is caused to spread in proportion to the amplitude of the conjoint voltages, the charge spread establishing the spatial size of the spot where the liquid crystal material changes state thereby providing spatial gray level of light transmissive point at selected matrix cross-points.
Color filters applied to the transparent electrodes provide a color display. It will be appreciated that the color filter can be conventional red, blue, green color filters with the size of the light spot or area of each light spot being adjusted to produce various combinations and hues of colors from the conventional three-color combination.
Alternatively, a color display can be achieved using the LCD active AC plasma assembly in a projection display in which synchronized color backlight is used in place of front color filters. In this embodiment, the active AC plasma panel provides in addition to eliminating the front color filtering, reduces the column drive electronics, has easier physical construction and manufacturing, and has a greater pixel fill factor and the active AC plasma panel provides a thermal buffer between the projection light sources and the LCD portion of the display.